Fuel cells operate by electrochemical oxidation of hydrogen or hydrocarbon fuels at an anode and the reduction of oxygen at a cathode. Fuel cells are attractive power sources due to their high conversion efficiencies, little or no pollution, light weight, and high power density. Extensive studies have been focused on using platinum-group (Pt-group) metals as materials for both anode and cathode catalysts. However, due to the sluggish reaction kinetics, particularly at the cathode, the efficiency and fuel economy of fuel cells using these catalysts have been limited. Moreover, because of the generally high cost of Pt-group metals, their use in fuel cells has also been limited. It has been necessary to either replace them with cheaper catalysts or to reduce the amount of Pt-group metals used.
Though nanoparticle platinum-group catalysts are well known, practical methods for the preparing platinum-based alloy nanoparticles, (e.g. in the 2–3 nm size range) have heretofore not been available. Such nanoparticles can solve some of the problems of the prior art by increasing the intrinsic kinetic activity of catalysts formed therefrom. Furthermore, as the size of the particles is reduced, the ration of surface area to volume increases. This ratio increase results in a higher utilization of the catalysts. However, it is a major challenge to synthesize and process alloy nanoparticles of such a small size (e.g., approximately 2 nm) with high monodispersity and controlled composition. Most existing approaches to producing such nanoparticles involve deposition and co-precipitation. These prior art processes, unfortunately, can not produce alloy nanoparticles of such a small size range (e.g., 2 nm) with the high monodispersity and controlled phase and composition required for efficient fuel cell catalysts. In addition, when the nanoparticles prepared using such prior methods are supported on carbon, the particles have not been highly dispersed or very uniform.
The preparation methods of the present invention, however, overcome the disadvantages of the preparation methods of the prior art. The inventive approaches are based on the use of core-shell gold and gold-based alloy nanoparticles as described in technical papers, references: S. H. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, “Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices”, Science, 2000, 287, 1989; M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, “Synthesis of Thiol-Derivertized Gold Nanoparticles in a Two-Phase Liquid—Liquid System”, J. Chem. Soc., Chem. Comm. 1994, 801; Zhong, C. J.; Zheng, W. X.; Leibowitz, F. L.; Eichelberger, H. H., “Size and Shape Evolutions for Thiolate-Encapsulated Gold Nanocrystals”, Chem. Commun., 1999, 13, 1211; M. M. Maye, W. X. Zheng, F. L. Leibowitz, N. K. Ly, C. J. Zhong, “Heating-Induced Evolution of Thiolate-Encapsulated Gold Nanoparticles: A Strategy for Size and Shape Manipulations”, Langmuir, 2000, 16, 490; M. M. Maye, C. J. Zhong, “Manipulating core-shell reactivities for processing nanoparticle sizes and shapes”, J. Mater. Chem., 2000, 10, 1895; C. J. Zhong, M. M. Maye, “Core-Shell Assembled Nanoparticles as Catalysts”, Adv. Mater., 2001, 13, 1507.